In recent years, there has been a rapid advancement of research and development of a high density disc system capable of recording and/or reproducing (hereinafter, “recording and/or reproducing” will be described as “recording/reproducing”) information by using a violet semiconductor laser with a wavelength of about 400 nm. Comparing with DVD which is an optical disc having a diameter of 12 cm with specifications of NA 0.6, light source wavelength of 650 nm and memory capacity of 4.7 GB, one example of the high density disc system records/reproduces information with specifications of NA 0.85 and light source wavelength of 405 nm for an optical disc, so-called Blu-ray Disc (hereinafter referred to as BD). In the optical system, information of 23-27 GB per one layer can be recorded for the optical disc having a diameter of 12 cm identical in terms of size to DVD. Further, another example of the high density disc system records/reproduces information with specifications of NA 0.65 and light source wavelength of 405 nm to an optical disc, so-called HD DVD (hereinafter referred to as HD). In the optical system, information of 15-20 GB per one layer can be recorded for the optical disc having a diameter of 12 cm. Incidentally, an amount of coma caused by an inclination (skew) of the optical disc increases in BD. Therefore a protective layer of BD is designed to be thinner than that in DVD (where the thickness is 0.1 mm in BD, as against 0.6 mm in DVD) to reduce an amount of coma caused by skew. In the present specification, these optical discs of this kind are called “high density disc”. On the other hand, as for HD, a thickness of a protective layer is made to be 0.6 mm in the same way as in DVD.
When considering the present situation where DVD and CD (compact disc) on which various information are recorded are on the market, it is desired that a single player can record/reproduce information properly for optical discs in as many types as possible.
When compatibly recording/reproducing information by using a single optical pickup apparatus for plural types of optical discs, there is a problem caused by NA of an objective lens that is different for each optical disc. For example, when recording/reproducing information for an optical disc having a specification of large NA, all light fluxes within an effective diameter corresponding to the NA (which is named as NA1) need to be converged on an information recording surface of an optical disc. On the other hand, when recording/reproducing information for an optical disc having specification of NA that is smaller than the aforesaid NA, which is named as NA2, it is necessary to make a photodetector to detect nothing by controlling a light flux passing through the portion outside an effective diameter which corresponds to NA2. To Control the light passing through the portion outside an effective diameter is called aperture controlling.
As a method of realizing this aperture controlling, it is considered that a diffractive optical element forms a light flux that passes through an outer portion of the effective diameter corresponding to NA2 into a flare light on the information recording surface of an optical disc, as disclosed in Japanese Patent Publication Open to Public Inspection (JP-A) No. 2001-195769.
However, when a saw-toothed diffractive structure in a form of concentric circles described in JP-A No. 2001-195769 is used to generate flare light, the structure for that turns out to be complicated. It needs highly-qualified technology for forming a microscopic structure on a molding transfer surface of a die, to cope with that complicated structure. Further, even when such die can be formed, it still needs technology at a high level for forming a highly accurate diffractive structure on an optical element, thus, synergism of the foregoing caused notable cost increase, which has been a problem.